1. Field of the Invention
The present invention relates to a plasma reactor for performing gas modification reaction so as to synthesize or decompose gas, and to a method for modifying gas. More particularly, the invention relates to a plasma reactor and a method for modifying gas for performing gas modification reaction with high efficiency by employing complex plasma discharge.
2. Background Art
Conventionally, there have been known gas modification methods employing discharge.
An example of the known methods is a method for plasma-treating a contaminant gas of a harmful substance; e.g., NOX, VOC (volatile organic compound) gas, or ethylene by employing silent discharge so as to purify the gas.
The aforementioned silent discharge is a type of discharge which is attained by applying AC high voltage to two planar electrodes which face opposite each other and which sandwich a dielectric layer formed of an insulating substance. The silent discharge uniformly disperses between the electrodes even at ambient pressure.
Among the methods for modifying gas by plasma, typically employed methods are categorized, in accordance with the nature of the plasma induced between the electrodes, into the following two types:
1. Gas modification methods employing localized and concentrated discharge such as corona discharge, glow discharge, or arc discharge, which is induced by applying voltage between a pair of electrodes facing opposite each other, and
2. Gas modification methods employing barrier discharge, which is induced by forming a dielectric body on at least a metallic electrode surface and, subsequently applying voltage between the electrodes.
Japanese Patent Application Laid-Open (kokai) No. 6-106025 discloses an exhaust-gas-purifying apparatus for removing NO contained in exhaust gas. The exhaust-gas-purifying apparatus employs an exhaust-gas-purification catalyst and a plasma reactor in combination. In fact, there are disclosed (ibid.) one apparatus employing a plasma reactor in which lightning-like concentrated discharge is induced through the application of AC voltage between a pair of electrodes, and another apparatus employing a plasma reactor in which barrier discharge is induced by applying AC voltage between a pair of electrodes, at least one of which is coated with a dielectric body.
However, concentrated discharge of high plasma energy density disadvantageously attains contact with a reaction gas at low probability. In contrast, barrier discharge that attains contact with reaction gas at high probability has a disadvantageously low plasma energy density.
In view of the foregoing, the present inventors have conducted extensive studies in an effort to elevate the plasma energy level over a region between the electrodes, and have found that the collision frequency of molecules of a gas introduced for treatment can be enhanced by complex barrier discharge; i.e., combination of mist-like barrier discharge and lightning-like localized and concentrated discharge, to thereby enhance the gas reaction efficiency.
Accordingly, in one aspect of the present invention, there is provided a plasma reactor for modifying gas by plasma, characterized by comprising
a first planar electrode and a second planar electrode, the two electrodes facing opposite each other approximately in parallel;
a dielectric body inserted between the first and the second electrodes; and
a complex barrier discharge-generating means for providing a predetermined electric potential difference between the first and the second electrodes; wherein the first and the second electrodes are provided so as to apply complex plasma discharge to the gas to be treated fed between the electrodes, to thereby modify the gas.
The ratio of the width (W) to the length (L) of the first and second electrodes may be predetermined in accordance with modification reaction of the gas to be treated, the width (W) being approximately perpendicular to the direction for feeding the gas to be treated and the length (L) being along the direction.
The relationship between W and L may be adjusted to Wxe2x89xa7L when the modification reaction is a single-step reaction, or the relationship between W and L may be adjusted to Wxe2x89xa6L when the modification reaction includes multiple reaction steps.
Positions of voltage application to the first and the second electrodes may be offset from a central position with respect to the direction of the flow of the gas to be treated.
The positions of voltage application to the first and the second electrodes may differ from each other with respect to the direction of the flow of the gas to be treated.
The reactor may be provided for treatment of a gas of a substance which has a low dissociation energy and can be decomposed by low-density plasma.
The reactor may be provided for treatment of NOX.
The positions of voltage application to the first and the second electrodes may be identical to each other with respect to the direction of the flow of the gas to be treated; face opposite each other; and are offset upstream from a central position with respect to the direction of the flow of the gas to be treated.
The reactor may be provided for treatment of a gas of a substance which has a high dissociation energy and can be decomposed by high-density plasma.
The reactor may be provided for treatment of CO2 fed to the reactor.
A plurality of projections may be formed on one or both surfaces of the dielectric body.
A plurality of units may be stacked, the units being formed from the first and the second electrodes and the dielectric body inserted between the electrodes.
The units may adjacent to each other share at least one electrode.
The projections formed on the surface of the dielectric body may have a cross-sectional shape selected from the group of a rhombus, a polygon, a circle, and an ellipse.
The projections formed on the surface of the dielectric body may be of different heights.
The dielectric body may be not in contact with at least one of the first and the second electrodes.
The dielectric body may be in contact with the first and the second electrodes.
Metallic microparticles may be dispersively deposited on the surface of the first electrode, to thereby induce complex barrier discharge through the application of high voltage.
The dielectric body may be stacked on the surface of the second electrode.
The metallic microparticles may have a high thermoelectron-emission property.
The metallic microparticles may be formed of at least one metal selected from the group consisting of tungsten, platinum, thallium, niobium, nickel, zirconium, cesium, and barium.
The metallic microparticles may have a high secondary-electron-emission property.
The metallic microparticles may provide a small glow-cathode-fall-voltage and have a high secondary-electron-emission property.
The metallic microparticles may be formed of at least one species selected from a group consisting of magnesium oxide, cesium-containing material, copper-beryllium, silver-magnesium, rubidium-containing material, and calcium oxide.
The metallic microparticles may be dispersed in a uniform manner or a localized manner.
The surface coverage by the dispersively deposited metallic microparticles may be 20-60%.
In another aspect of the present invention, there is provided a method for modifying gas by plasma, characterized by comprising
feeding the gas to be treated into a space between the first and the second electrodes, and
applying complex plasma discharge to the gas, to thereby cause gas modification reaction, the plasma being provided by a plasma reactor comprising a first planar electrode and a second planar electrode, the two electrodes facing opposite each other approximately in parallel; a dielectric body inserted between the first and the second electrodes; and a complex barrier discharge-generating means for providing a predetermined electric potential difference between the first and the second electrodes.
The ratio of the width (W) to the length (L) of the first and second electrodes may be predetermined in accordance with modification reaction of the gas to be treated, the width (W) being approximately perpendicular to the direction for feeding the gas to be treated and the length (L) being along the direction.
The relationship between W and L may be adjusted to Wxe2x89xa7L when the modification reaction is a single-step reaction, or the relationship between W and L may be adjusted to Wxe2x89xa6L when the modification reaction includes multiple reaction steps.
High voltage may be applied, and the positions of voltage application to the first and the second electrodes are offset from a central position with respect to the direction of the flow of the gas to be treated.
High voltage may be applied, and the positions of voltage application to the first and the second electrodes differ from each other with respect to the direction of the flow of the gas to be treated.
High voltage may be applied, and the positions of voltage application to the first and the second electrodes are identical to each other with respect to the direction of the flow of the gas to be treated; face opposite each other; and are offset upstream from a central position with respect to the direction of the flow of the gas to be treated.
Metallic microparticles may be caused to be dispersively deposited on the surface of at least one of the first and second electrodes, to thereby induce complex barrier discharge through the application of high voltage.
In order to induce complex plasma discharge, there must be appropriately set conditions such as dielectric constant of the dielectric body inserted between the electrodes, the mode for placing the dielectric body, the shape of the dielectric body, the distance between the electrodes, and voltage to be applied to the electrodes.
Particularly, complex barrier discharge can be obtained at high efficiency by employing the aforementioned preferred modes of the plasma reactor.
The type of complex barrier discharge to be induced is preferably modified in accordance with conditions such as the species of the gas to be treated and the type of reaction for gas modification.
For example, as described above, the probability of contact between plasma and modified gas molecules can be controlled by modifying the dimensions of the gas passage between the electrodes for generating plasma in accordance with the gas to be treated. Specifically, the ratio of the width to the length (W/L) is predetermined in accordance with the type of gas modification reaction. Thus, the cumulative excited state of reaction gas molecules can be controlled, to thereby enhance selectivity of modification products and modification efficiency.
In addition, by offsetting the positions of high-voltage application to the electrodes from a central position with respect to the gas flow direction, the electric field profile between the electrodes is modified, thereby attaining high-efficiency and high-selectivity gas modification.
Thus, an object of the present invention is to provide a plasma reactor attaining remarkably enhanced gas modification efficiency. Another object of the invention is to provide a method for modifying gas attaining remarkably enhanced gas modification efficiency.
In the present invention, the plasma reactor and the gas modification method are adjusted in accordance with reaction steps of the gas to be treated.
In addition, the plasma reactor and the gas modification method attain high-efficiency and high-selectivity gas modification by controlling the electric field profile between the electrodes. dr
Various other objects, features, and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood with reference to the following detailed description of the preferred embodiments when considered in connection with accompanying drawings, in which:
FIG. 1 is a schematic view of a plasma reactor employed in Embodiment 1 of the present invention;
FIGS. 2A and 2B are graphs showing results of Test Example 1 of Embodiment 1 of the present invention;
FIGS. 3A and 3B are graphs showing results of Test Example 2 of Embodiment 1 of the present invention;
FIG. 4 is a schematic view of a plasma reactor employed in Embodiment 2 of the present invention;
FIG. 5 is a map showing electric field distribution of the apparatus shown in FIG. 4;
FIG. 6 is a schematic view of a plasma reactor employed in Embodiment 3 of the present invention;
FIG. 7 is a map showing electric field distribution of the apparatus shown in FIG. 6;
FIG. 8 is a schematic view of a test apparatus employed in Test Examples;
FIGS. 9A to 9D are schematic views of a test apparatus having different voltage application positions;
FIG. 10 is a graph showing results of Test Example 3 and the relationship between the voltage application position and the decomposition rate of NOX;
FIG. 11 is a map showing electric field distribution of the apparatus shown in FIG. 9C;
FIG. 12 is a graph showing results of Test Example 4 and the relationship between the voltage application position and the decomposition rate of CO2;
FIG. 13 is a schematic view of a plasma reactor employed in Embodiment 4 of the present invention;
FIGS. 14A to 14C are schematic views of patterns of projections;
FIG. 15 is a schematic view of a plasma reactor employed in Embodiment 5 of the present invention;
FIG. 16 is a schematic view of a plasma reactor employed in Embodiment 6 of the present invention;
FIG. 17 is a schematic view of a plasma reactor employed in Embodiment 7 of the present invention;
FIGS. 18A to 18C are schematic views of patterns of projections employed in Test Examples;
FIGS. 19A to 19C are schematic views of patterns of projections employed in Test Examples;
FIGS. 20A to 20C are schematic views of patterns of projections employed in Test Examples;
FIG. 21 is a graph showing the decomposition rates of CO2 obtained in Test Examples;
FIG. 22 is a graph showing the decomposition rates of NOX obtained in Test Examples;
FIG. 23 is a schematic view of a plasma reactor employed in Embodiment 8 of the present invention;
FIG. 24 is a perspective view of a metallic electrode on which metallic microparticles are dispersively deposited;
FIG. 25 is a graph showing the relationship between the surface coverage of metallic microparticles having a high thermoelectron-emission property and the decomposition rate of NOX;
FIG. 26 is a graph showing the relationship between the surface coverage of metallic microparticles having a high thermoelectron-emission property and the decomposition percentage of CO2;
FIG. 27 is a graph showing the relationship between the particle size of metallic microparticles having a high thermoelectron-emission property and the decomposition percentage of CO2;
FIGS. 28A and 28B show dispersion states of metallic electrodes on which metallic microparticles have been dispersively deposited; and
FIG. 29 is a graph showing the relationship between the surface dispersion state of metallic microparticles of high thermoelectron-emission property and the decomposition percentage of CO2.